In CT scanning it is desirable to be able to clearly distinguish between different materials. Indeed, the ability of CT systems "to discriminate between tissues with very slightly different X-ray absorption coefficients" contributes significantly to their success. R. A. Rutherford, et al., "Quantitative Aspects of Computer Assisted Tomography", p. 605, Proceedings of The British Institute of Radiology, July 1975. The ability to differentiate between different substances is a function of many factors. Important among these are the measurement techniques used and noise factors. Measurement techniques may distinguish among materials on the basis of factors such as density or chemical composition. Thus, materials which may be closely related in density may be distinguishable from each other on the basis of differing chemical composition. However, the differences between measurements may be so slight that they are easily masked by quantum noise in the system. In CT scanning this is frequently the case, where noise is great enough to make materials close in density difficult to differentiate.
The ability to distinguish between different materials in CT scanning is also a function of the energy level employed. CT scanners measure materials by detecting differences in energy attenuation, a function of density. Two materials may exhibit the same attenuation coefficients at 80 KeV (thousand electron volts), but may differ significantly in attenuation coefficients at 40 KeV. This is typically true of white and grey brain matter, a frequent subject of CT scanning and a common standard for the ability of a CT scanner to discriminate between materials close in density. A relatively low KeV scan is desirable for brain matter so that the two materials are readily distinguishable in the resultant image. But at these low energy levels, the flux level of conventional radiation sources used in CT scanning is generally insufficient to produce clearly distinguishable images in the presence of noise. Sufficient flux can usually only be obtained by using a higher KeV level with its broader energy spectrum and higher dose level. Thus a dilemma is present when a high flux level is desired with a low KeV scan.
Dual energy scanning systems have been suggested as the solution to many of these problems where two scans are made at a combined dose equal to the dose that would have been used if the single energy scan approach had been used. By taking two sets of measurements, one at a high KVP (kilovolts peak) at a specified dose level and another at a low KVP and at a specified corresponding dose level, information may be obtained from which estimates may be made about the distribution functions of attenuation coefficients at a given reconstruction energy. Given a low KVP energy spectrum and a high KVP energy spectrum, attenuation coefficient values may be obtained for each pixel in the displayed image. But the effects of noise differ for the two energy levels, and are most pronounced at the lower energy level. It is desirable, then, to be able to reduce the effects of noise on the reconstructed dual energy image, particularly at the lower of the two energy levels. See, for example, U.S. Pat. No. 4,029,963,issued Jun. 14, 1977, to Alvarez, et al. which suggests obtaining projection measurements of a CT X-ray transmission at low and high energies and reconstructing from such measurements two separate cross-sectional images one of which is suggested to be energy-independent. To date, however, there is not a single commercially available CT scanner designed for dual energy scanning.